Polyethylene has traditionally been used as a sealing layer in polyolefin films and packaging because of its ability to seal quickly and maintain a strong seal strength. Typically polyethylene has been coextruded, laminated or otherwise bonded to other polyolefins which have better strength than polyethylene, yet do not seal as well. For example, in a typical multilayer films, a polypropylene layer, especially a mono or biaxially oriented polypropylene (OPP) layer, provides a high clarity, high melting, high barrier properties, combined with high stiffness, while a polyethylene layer will provide extra body to the film and will allow a low sealing temperature, meaning higher packaging speeds. However, polypropylene (PP) and polyethylene (PE) have very limited compatibility and direct sealing of polyethylene onto polypropylene film is not commonly done. When a layer of PE is combined with a layer of PP, extra primer may be needed. For example, extra primer is used when polyethylene, such as low density polyethylene, is coated onto polypropylene films. In addition tie layers may also be necessary. Coextrudable tie layers such as ethylene vinyl acetate copolymers, typically having more that 8 weight % vinyl acetate, have been extruded between PP and PE to enhance adhesion between the PE and the PP.
Furthermore, since polyethylene does not seal well to polypropylene, the sealing operation for films containing PE and PP layers typically has to be performed inside versus inside. This causes difficulties in traditional "form, fill and seal" applications. During form, fill and seal operations, a first transverse seal is made to shape the flat film into a tube. This tube is then transported to the top/bottom sealing bars, where the actual bags will be made. Thereafter a bottom seal is made, the bag is filled and a top seal is made. In the past when polyethylene/polypropylene film has been used in form, fill and seal applications, the transverse seal had to be made between the two inside surfaces of the tube. This seal was then typically folded to produce a flat tube which thereafter resulted in the formation of a "thick spot" where the bottom and top seals were made.
(At the transverse seal four film layers were between the sealing bars, while at the other sealing areas, only two film layers were between the sealing bars.
These thick spots, where the transverse seals cross the top and bottom seals, are typically weak points in the packaging and are particularly undesirable in applications requiring perfect seals, such as Modified or Controlled Atmosphere Packaging (MAP/CAP). However if the transverse seal could be made by sealing the inside polyethylene layer to a polypropylene outside layer (or vise versa) and still obtain a high performing seal, the number of weak spots (i.e. leaks) would be reduced immediately and the overall packaging speed could be increased with greater packaging quality.
Likewise, since oriented polypropylene/polyethylene film structures are typically used to package small quantities of various goods, (e.g. 300 grams of nuts, 500 grams of candy, etc.) a significant material savings could be obtained if the polyethylene layer could be sealed directly onto the oriented polypropylene outer layer.
Therefore there is a need in the art to provide a means to seal polyethylene to polypropylene, particularly oriented polypropylene.
WO 95/00333 discloses heat shrinkable films where an outer layer of a multilayer heat shrinkable film is an ethylene or propylene polymer formed by polymerization in the presence of a single site catalyst. EXACT.TM. resins are disclosed as suitable as the outer layer, but traditional polypropylene is not disclosed as a suitable second surface layer.
EP Patent Application 0 597 502 discloses oriented single and multilayer films of metallocene polyethylenes, however polypropylene is not disclosed as a layer.
U.S. Pat. No. 5,482,770 discloses irradiated oriented multilayer films having a barrier layer of EVOH and other core or sealayers of metallocene polyethylenes. Polypropylene is not disclosed as a layer.
Art disclosed for U.S. purposes includes U.S. Pat. Nos.: 5,530,065; 5,397,613; 5,376,439; 5,358,792; 5,206,075; and PCT publications WO 95/21743; WO 95/16729; WO 95/15851; WO 94/26816; WO 94/25271; WO 94/06857.